To improve gear accuracy, an internal-gear workpiece having undergone a heat treatment is secured to a mount table, and the tooth surface of the internal-gear workpiece in this state is finished by grinding. Examples of a method for securing the internal-gear workpiece include a method in which an outer circumferential surface of the internal-gear workpiece is clamped and a method in which an end surface of the internal-gear workpiece is clamped (see, for example, Patent Document 1).
As the method in which the outer circumferential surface of the internal-gear workpiece is clamped, there is a method using a collet or multiple claw portions. In the method using a collet, a workpiece is placed on an upper surface of a mount table, and a collet is arranged to hold the outer circumferential surfaces of the workpiece and the mount table. Then, the collet is drawn downward to forcibly reduce the diameter of the collet near the upper end thereof. Thus, the workpiece is held at the entire circumference of its outer circumferential surface, and is secured to the mount table. In the method using multiple claw portions, a workpiece is placed on an upper surface of a mount table, and the multiple claw portions are pressed against an outer circumferential surface of the workpiece. Thus, the workpiece is held at its outer circumferential surface, and is secured to the mount table.
Examples of the method in which the end surface of an internal-gear workpiece is clamped include a method using fingers or arms. In the method using fingers, a workpiece is placed on an upper surface of a mount table, and multiple fingers are turned vertically to press tip end portions of the fingers against an end surface of the workpiece. Thus, the workpiece is held at its end surface, and is secured to the mount table. In the method using arms, a workpiece is placed on an upper surface of a mount table, and multiple arms are turned horizontally to place tip end portions of the arms on an end surface of the workpiece, and base end portions of the arms are drawn downward. Thus, the workpiece is held at its end surface, and is secured to the mount table.